1. Field of the Invention
The present invention relates to an apparatus and method for cleaning, on a one-by-one basis, precision substrates such as semiconductor substrates, glass substrates for use in liquid crystal displays, and magnetic disks. Particularly, the invention relates to cleaning of a precision substrate through use of cleaning liquid to which high frequency or ultrasonic vibration is applied.
2. Description of the Related Art
Precision substrates such as semiconductor substrates and quartz substrates are used in the manufacture of semiconductor devices, liquid crystal displays, magnetic disks, and the like. A recent tendency to increase the degree of integration and precision of such devices has been accompanied by increasing demand for higher cleanliness of these substrates. Accordingly, improvement of technique for cleaning precision substrates is an essential factor for improving the quality and yield of advanced devices in their future manufacture.
Also, along with a recent tendency not only to improve the precision of precision substrates such as semiconductor substrates but also to increase the diameter and area of precision substrates, such precision substrates tend to be cleaned one by one rather than by batch cleaning in which a plurality of substrates are concurrently cleaned in a cleaning bath or the like.
According to a conventional apparatus or method for cleaning a single precision substrate, cleaning liquid to which high frequency or ultrasonic vibration is applied (hereinafter referred to as "high-frequency- or ultrasonic-applied cleaning liquid") is jetted from above onto the surface of a rotating object substrate.
This method has the following advantages: since high frequency or ultrasonic vibration is applied to cleaning liquid, cleaning is efficient; cleaning power is improved by increasing the jetting pressure of cleaning liquid; and since unlike the case of the above-mentioned method using a cleaning bath a substrate is always cleaned with new cleaning liquid, recontamination of the substrate with cleaning liquid does not occur.
Examples of a nozzle for jetting cleaning liquid are shown in FIGS. 2A and 2B. The nozzle shown in FIG. 2A has the shape of a truncated cone, and that shown in FIG. 2B has the shape of a trapezoid.
In a nozzle 41 having the shape of a truncated cone as shown in FIG. 2A, a vibrator 42, whose high frequency or ultrasonic generation surface has a circular shape, is attached to one end of the nozzle 41, and a cleaning liquid inlet port 43 is provided on the side surface of the nozzle 41. Cleaning liquid fed through the inlet port 43 is jetted from an opening 44 located at the other end of the nozzle 41 while high frequency or ultrasonic vibration is applied to the cleaning liquid. Through employment of this arrangement, an unillustrated object substrate can be cleaned intensively.
In a nozzle 41 having the shape of a trapezoid as shown In FIG. 2B, a vibrator 42, whose high frequency or ultrasonic generation surface has an elongated rectangular shape, is attached to one end of the nozzle 41, and cleaning liquid inlet ports 43 are provided on the side surfaces of the nozzle 41. Cleaning liquid fed through the inlet ports 43 is jetted from an opening 44 located at the other end of the nozzle 41 while high frequency or ultrasonic vibration is applied to the cleaning liquid. As compared with a truncated cone nozzle, the trapezoidal nozzle 41 can clean an object substrate over a wider area.
A nozzle of the type as shown in FIG. 2A (called a jet nozzle) has high cleaning power because of a high jetting pressure of cleaning liquid. However, since the area of cleaning covered by the nozzle is relatively small, the nozzle must be moved in a scanning manner or a like manner in order to jet cleaning liquid over the entire surface of an object substrate. Thus, cleaning a single substrate takes a relatively long time, resulting in a reduction in productivity. Further, when the nozzle is situated above the circumferential portion of a rotating substrate, cleaning liquid on the central portion of the substrate is centrifuged toward the circumference of the substrate. Thus, the thickness of liquid film on the central portion becomes thin, causing difficulty in transmission of high frequency or ultrasonic vibration to the central portion. As a result, the central portion is not cleaned effectively. In some case, no liquid film is present on the central portion, which thus becomes dry and contaminated.
By contrast, because of a relatively large area coverage of cleaning, a nozzle of the type as shown in FIG. 2B (called a squall nozzle) can readily jet cleaning liquid over the entire surface of a substrate and does not necessarily require scanning movement. Accordingly, this type of nozzle cleans a single substrate within a relatively short period of time and is free from the above-mentioned problem that the film of cleaning liquid becomes thin on the central portion of a substrate due to the effect of a centrifugal force with a resultant difficulty in transmission of high frequency or ultrasonic vibration to the central portion and is also free from the problem that due to the effect of a centrifugal force, no liquid film is present during cleaning on the central portion, which thus becomes dry and contaminated.
However, because of a lower jetting pressure of cleaning liquid, the squall nozzle is inferior to the jet nozzle in terms of cleaning power. Further, the squall nozzle is disposed above an object substrate, and a cleaning liquid jetting port is located under a high frequency or ultrasonic vibrator. Accordingly, bubbles generated within the squall nozzle are accumulated in the upper portion of the nozzle, i.e. just under the vibrator. As a result, high frequency or ultrasonic vibration becomes difficult to transmit to cleaning liquid, resulting in a reduction in cleaning power. To prevent this problem, cleaning liquid must be jetted at or above a certain jetting rate. As a result, not only does the consumption of cleaning liquid increase, but also cleaning liquid is jetted in a greater amount than required. This brings about a cost disadvantage.